Comparison of Thermodynamic Equilibrium Predictions on Trace

May 29, 2014 - The importance of coal as the main energy source has posed the ... use this fuel in a sustainable way while new clean energy technologi...
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Comparison of Thermodynamic Equilibrium Predictions on Trace Element Speciation in Oxy-Fuel and Conventional Coal Combustion Power Plants Marco A. Jano-Ito, Graham P. Reed, and Marcos Millan* Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ United Kingdom ABSTRACT: The importance of coal as the main energy source has posed the challenge to use this fuel in a sustainable way while new clean energy technologies are developed. Carbon capture and storage is one of the main short-term solutions to the problem of CO2 emissions and involves a wide range of technologies. Oxy-fuel combustion is one of them, and even though it has emerged as an attractive alternative, a better understanding of the performance of the process is still required. The current work was aimed at enhancing the understanding of the different thermodynamic equilibrium behaviors of metallic trace elements in coal combustion under air and oxy-fuel environments. The effect of coal composition on the speciation of these elements was analyzed, and a sensitivity analysis of the effect of various concentrations of Cl, S and the mineral content of coal was performed. It was found that the increase in the concentration of CO2 and H2O did not have a significant effect on the thermodynamically stable forms of trace element compounds that were predicted to form. The recycling of a larger amount of trace elements did not affect the speciation either but only increased the concentration of trace elements inside the boiler. The speciation and volatility of the species predicted by thermodynamic modeling in the majority of the trace elements considered in this study was found to be sensitive mainly to changes in the concentration of Cl, Ca, S, and Si. In terms of species that may enhance corrosion at tube wall temperatures, the thermodynamic calculations predicted the condensation of the majority of trace elements as sulfates as well as the formation of V2O5.

1. INTRODUCTION Current electricity generation processes rely to a great extent on the use of low cost and easily available fossil fuels.1 However, CO2 release into the atmosphere importantly contributes to the observed global warming.2 The growing trend in the use of fossil fuels, particularly coal, which is attractive because of its low cost, extensive distribution around the world, and reserves expected to last for several hundreds of years, is not likely to change in the short term.3 For this reason, mankind faces the enormous challenge to secure its energy future and reduce CO2 emissions. Carbon dioxide capture and storage (CCS) technology provides an alternative which will separate CO2 emissions from fossil fuel use in the near term and enable the long-term transition to cleaner energy sources that cannot meet today’s energy demand.4 Oxy-fuel combustion is one of the promising technologies that have been proposed as an alternative for burning coal with capture of CO2 emissions. The fate of trace elements in oxy-fuel combustion has received little attention, and the number of studies available is very limited. However, the following paragraphs summarize the existing theoretical and experimental work. Zheng and Furimsky5 worked on thermodynamic equilibrium calculations of coal combustion in the mixture O2/CO2 and O2/N2. The calculations were performed for different temperatures and concentrations of CO2 using the composition of a U.S. eastern bituminous coal, which was experimentally evaluated by Croiset, Thambimuthu, and Palmer6 at CANMET’s pilot plant facility. The authors analyzed the combustion behavior of As, Pb, Hg, Cd, Se, Cl, and alkali as well as the fate of CO, NOx, SOx, and ash formation and composition. The amounts of As and Pb used in the calculations were similar to those found in Eastern © 2014 American Chemical Society

Canadian coals whereas for Hg, Cd, and Se these values were determined arbitrarily. The emission of CO was higher for oxy-fuel combustion, while NOx was lower. It was found that trace element speciation and concentration in the gaseous phase were not affected by the combustion environment or by the ash composition. It was reported that elements present in ash such as Ca, Na, K, Fe, and Mg formed sulfates rather than carbonates.5 Jiao et al.7 experimentally and theoretically analyzed the oxyfuel combustion of a brown coal at 1000 °C in order to study the effects of impurity recirculation in flue gases (H2O, HCl, and SO2) on the vaporization of metals that are associated with the organic matrix and ash particles, focusing on the speciation of Na. Vaporization of metals (Na, Mg, Ca, Al, Fe, K) was observed when the ratio SO2/HCl was lowered. It was also found that Na was the main element affected by an increase in SO2 concentration. The other metals showed little sensitivity to an increased concentration of SO2, indicating the lack of oxides to take SO2 and form sulfates.7 A special emphasis has been put on Hg. Gharebaghi et al.8 developed a homogeneous-heterogeneous kinetic model of the transformation of Hg in air-fired and oxy-fuel combustion and found a higher amount of particulate mercury in the case of oxy-fuel combustion.8 Another study9 focused on speciation of this element to submicrometer particle formation in O2/CO2 and O2/N2 environments. The authors concluded that the amount of mercury found in the elemental form was around 4 times greater than in the oxidized form. It was also found Received: November 6, 2013 Revised: May 22, 2014 Published: May 29, 2014 4666

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oxy-fuel configurations with 72.1% and 55.9% by mass recycle of flue gases. The base cases in this work consisted in the three systems described above using a coal with the composition of Illinois No. 6 (Tables 1 and 2).19,20

that CO2 did not have an effect in the vaporization of mercury and that its concentration was similar in the air-fired experiment.9 More recent work10 used thermodynamic equilibrium calculations to study the speciation behavior of Cr, As, Se, and Hg in air-fired and oxy-fuel systems, firing three Victorian brown coals. The authors found that the concentration of gaseous hexavalent Cr was higher for oxy-fuel combustion, while As and Se presented almost the same speciation behavior for the air-fired and oxy-fuel systems. Gaseous elemental Hg was predicted as the main form of Hg.10 The complexity and lack of detailed experimental data on the behavior of trace elements in oxy-fuel combustion makes it difficult to model these systems in terms of their reaction kinetics. Thermodynamic equilibrium calculations by minimization of the Gibbs energy provide an alternative approach that has been used for analyzing trace element behavior in conventional combustion and gasification systems11−18 and may give initial insights on the speciation of trace elements during oxy-fuel combustion. Nevertheless, this approach has limitations since it does not take into account mass transfer or chemical kinetic effects, in addition to partial mixing of elements, which are important in real combustion systems. In the particular case of practical pulverized coal combustion systems, the furnace zone of a boiler represents an area where very rapid gas−solid reactions take place approaching equilibrium. In the flame area of this zone, equilibrium does not exist because of temperature and/or concentration gradients. During combustion, trace elements end up as part of large ash particles, fly ash, or flue gas (vaporization). After the furnace and during cooling, condensable combustion products deposit on particles and some gaseous products (SO2, HCl, O2, and H2O) react with the deposited species without kinetic limitations.12 However, for trace elements, which are subject to complex physical and chemical transformations, gas−solid reactions may be kinetically constrained.14 In addition to this, and in the case of solid−solid reactions between deposited species and fly ash, diffusion limitations prevent equilibrium to be approached.11,12 Despite the fact that oxy-fuel combustion is considered as an attractive alternative for CO2 capture, the early stage of implementation of this technology needs a further understanding of its physical and chemical behavior. Trace element speciation and release is one of the areas of oxy-fuel combustion that is of immense importance because of operability issues related to potential deposit formation and corrosion of process equipment. The work that is presented in the following sections is aimed at providing a better comprehension of the fate of trace elements in oxy-fuel combustion through the use of chemical equilibrium calculations. Sections 2 and 3 introduce the systems under study and the methodology of the calculations, whereas section 4 presents the main results and discussion and a brief analysis of their relevance to deposition and corrosion.

Table 1. Composition of Illinois No. 6 Bituminous Coal19,20 proximate analysis moisture volatile matter ash fixed carbon total

as-received (%)

dry (%)

11.12 34.99 9.70 44.19 100.00

0.00 39.31 10.91 49.72 100.00

carbon hydrogen nitrogen sulfur chlorine ash moisture oxygen total

63.75 4.50 1.25 2.51 0.29 9.70 11.12 6.88 100.00 Typical Ash Mineral Analysis silica SiO2 aluminum oxide Al2O3 titanium dioxide TiO2 iron oxide Fe2O3 calcium oxide CaO magnesium oxide MgO sodium oxide Na2O potassium oxide K2O phosphorus pentoxide P2O5 sulfur trioxide SO3 undetermined total

71.73 5.06 1.41 2.82 0.33 10.91 0.00 7.74 100.00 45.0 18.0 1.0 20.0 7.0 1.0 0.6 1.9 0.2 3.5 1.8 100.0

Table 2. Average Composition of Trace Elements in Illinois No. 6 Coals20 trace element composition, dry basis, ppm trace element arsenic boron beryllium cadmium chlorine cobalt chromium copper fluorine mercury lithium manganese molybdenum nickel phosphorus lead antimony selenium thorium uranium vanadium zinc

2. SYSTEM DESCRIPTION The systems selected for this work were based on the design proposed by Seltzer, Fan, and Robertson19 in a report prepared for the United States Department of Energy (U.S. DOE). The report evaluated different configurations based on a 460 MW coal fired power plant, equipped with a supercritical steam turbine (main temperature and pressure conditions of 275 atm/588 °C, reheater temperature of 600 °C, and end pressure of 0.07 atm). The base case in the report corresponded to an air-fired system, which was also analyzed in the current work. The other two cases taken from the report were two 4667

As B Be Cd Cl Co Cr Cu F Hg Li Mn Mo Ni P Pb Sb Se Th Ur Vn Zn

arithmetic mean

standard deviation

7.5 90 1.2 0.5 1671 3.5 14 9.2 93 0.09 9.4 38 8.4 14 87 24 0.9 1.9 1.5 2.2 31 84.4

8.1 45 0.7 0.9 1189 1.3 6 2.5 36 0.06 7.1 32 5.7 5 83 21 0.7 0.9 0.4 1.9 16 84.2

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Figure 1. Air-fired process configuration. Adapted from ref 19. The red lines represent flue gases, and the blue lines represent the flow of steam, coal, and air. Temperatures (°C) of the flue gases and steam are indicated. recycled flue gases was set to 72.1% by mass in order to achieve the same adiabatic flame temperature of the air-fired case.19 Figure 2 presents a simplified diagram of the process where oxygen was separated from air in the air separation unit (ASU) and entered the air heater along with the recycled flue gases. This mixture was burned with coal and the products of combustion leaving the furnace passed through the same heat exchange equipment as in the air-fired case. Once the flue gases left the boiler, particulates were removed in an ESP, and the flue gas stream was divided into two streams (streams 3 and 2). The major part of the flue gases was present in stream 3, which was passed to a water separation unit. This unit produced a stream that was rich in CO2 (stream 5) and which was further divided into two streams (streams 7 and 6). Stream 7 was passed to a compression system which delivered the CO2 to be used for geological storage. Stream 6 was combined with stream 2, producing the recycled flue gases that entered the boiler (stream 8). The oxy-fuel system with a flue gas recycle of 55.9% (Figure 3) presented heat recovery units arranged in series. The reduction of the recycled flue gases increased the temperature of the combustion products, and as a consequence the equipment design was more compact than the one described in the previous cases.19 The heat balance was not affected, and the system produced 486.5 MW of gross power and a net power generation of 347 MW. The heat recovery area design eliminated the primary superheater, dividing the reheater duty in two units and increasing the heat duty of the finishing superheater.19 The information available from Seltzer, Fan, and Robertson19 was used to perform mass and energy balances for the three cases. The intention was to determine the amount of elements entering the boiler and the temperature of flue gases at different stages of the processes. Another important parameter that was estimated was the temperature of the wall of the tubes, which may play an important role in the

2.1. Air-Fired System. The air-fired process corresponded to a conventional supercritical pulverized coal system with a gross power of 476.2 MW and a net power of 430.2 MW. It operated at atmospheric pressure and had a parallel arrangement for heat recovery. This system did not incorporate carbon capture and storage technology but included flue gas cleaning devices. In the design presented by Seltzer, Fan, and Robertson,19 coal was introduced into the furnace where it was burned with air. The energy released from combustion was used to generate steam in the water walls and division walls of the furnace. Subsequently, the flue gases were used to heat steam in a finishing superheater and a finishing reheater. The flow of flue gases was then divided into two. A portion of them was used by a primary superheater and an upper economizer, whereas the other part was used by a primary reheater. The flue gases were mixed together and passed through a lower temperature economizer and a selective catalytic reduction (SCR) unit for nitrogen oxides (NOx) reduction. The heat from the flue gases was further used to preheat the incoming combustion air. The flue gases were finally cleaned by an electrostatic precipitator (ESP) and a flue gas desulfurization unit (FGD) and emitted to the atmosphere.19 The general arrangement of the air-fired system is illustrated in Figure 1. 2.2. Oxy-Fuel Combustion Systems. The oxy-fuel case with a flue gas recycle of 72.1% considered the same equipment arrangement for the heat recovery area as the air-fired case described previously but incorporated an air separation unit (ASU) for oxygen production and a CO2 compression system. In this case, the flue gas desulfurization (FGD) unit was not used. The gross power for this system was 476.2 MW, while the net power decreased to 331.7 MW compared to 430 MW for the air fired case, due to the energy requirement of the air separation unit (ASU) and the compression system. The amount of 4668

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Figure 2. Oxy-fuel process configuration for 72.1% flue gas recycle. Adapted from ref 19. The red lines represent flue gases, and the blue lines represent the flow of steam, coal, and air. Temperatures (°C) of the flue gases and steam are indicated. prediction and analysis of species that may condense and deposit on these surfaces, causing fouling and corrosion. The calculation procedure of the tube wall temperature consisted in the evaluation of the heat transfer coefficient inside the tubes and the heat conductivity of the materials used for the boiler as proposed by Ganapathy.21 Table 3 presents the obtained temperatures for the heat transfer surfaces.

3. MODELING OF TRACE ELEMENT SPECIATION The thermodynamic equilibrium model that was used to analyze trace element speciation was MTDATA version 4.81, a Gibbs free-energy minimization software developed by the National Physical Laboratory (NPL) in the United Kingdom, which allows the calculation of equilibrium in complicated multiphase multicomponent systems.22 The Multiphase module of MTDATA, which is based on the Scientific Group Thermodata Europe (SGTE) database, was used to perform the calculations. The module does not include mixing of condensed phases and therefore applies an ideal gas-phase model and a pure substance model for condensed phases.15 The simulations using MTDATA were carried out based on the information obtained from the mass balances for the inlet stream of the boiler and the temperatures from the energy balance. In order to perform the calculations, MTDATA required the elemental molar composition, a temperature range and the pressure of the system. The composition to be introduced to MTDATA was based on the major elements C, H, O, N; minor elements S, Cl, Ca, Si, Na, K and 15 trace elements, Sb, As, Be, B, Cd, Cr, Co, Cu, Pb, Mn, Hg, Ni, Se, V, and Zn. Trace elements were simulated individually with all the major and minor elements. As the number of elements to be modeled increases, the system increases in complexity and as a consequence the calculation becomes more time-consuming. MTDATA provides the option to select the species likely to be formed in the

Table 3. Average Temperature of Heat Transfer Walls heat recovery unit finishing superheater finishing reheater primary superheater primary reheater upper economizer lower economizer oxy-fuel system with 72.1% flue finishing superheater gas recycle finishing reheater primary superheater primary reheater upper economizer lower economizer oxy-fuel system with 55.9% flue finishing superheater gas recycle finishing reheater primary reheater lower economizer air-fired system

average wall temperature (°C) 585 601 476 497 342 319 585 601 477 497 341 320 598 628 488 314 4669

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Figure 3. Oxy-fuel process configuration for 55.9% flue gas recycle. Adapted from ref 19. The red lines represent flue gases, and the blue lines represent the flow of steam, coal, and air. Temperatures (°C) of the flue gases and steam are indicated.

(DOE)20 for the three base cases, air-fired and oxy-fuel with 72.1% and 55.9% flue gas recycle cases. In order to analyze the effect of higher or lower concentrations of Cl, S and the mineral content of coal on trace element speciation, a sensitivity analysis was performed. The methodology described previously was repeated for every base case and sensitivity case. The different sensitivity cases were defined on the basis of the highest and lowest concentrations of these elements found in coals from the United States and the ones produced in the Illinois region. The following concentrations were evaluated: (1) Chlorine. On the basis of the information found in the literature for the Cl content for coals in the United States, a concentration of 130 ppm (dry basis) was selected as the low Cl case.23 The content of Cl of the base case was already high (3300 ppm), and for this reason a high Cl case was not considered in the sensitivity analysis. (2) Sulfur. The content of S for Illinois coals can vary from 0.1% to 5.0% by mass, and these values were taken as the low S and high S cases, respectively.24 (3) Mineral content. Mineral matter contained in the coal (Ca, Si, K, Na were analyzed in this study) is defined as percentage of ash. For this reason, the amount of these elements was increased and decreased in the same manner, and the percentage of ash was defined to be 1.0% by mass for the low mineral case and 15% by mass for the high mineral case.24

system and limit the calculation to these compounds. In this study, in order to have a larger amount of elements in the system, the number of species that were allowed to form was reduced according to the following procedure. The calculations were based on 144.7 tons of coal for the air-fired case and 142.9 and 139.7 tons for the oxy-fuel cases (72.1% and 55.9%, respectively). These values corresponded to the amount of coal injected to the boilers per hour. The simulations were initially performed for the major and minor elements considering the complete database and from the results obtained from this initial assessment, the species that were present in more than 10−10 mol were selected as the species allowed to form in subsequent calculations.15 In the case of trace elements, all species in the database were allowed to form including the species from the initial assessment. With this regard, it has to be mentioned that the thermochemical data of many trace element species is extrapolated beyond their known validity. However, in this study, since a large temperature range was considered and because some species within this temperature range were predicted to form with no extrapolated and extrapolated data, it was decided to allow the formation of all species. The equilibrium calculations were also at first performed using the composition of coal given by Seltzer, Fan, and Robertson19 and the United States Department of Energy 4670

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Table 4. Flow Rate of the Main Trace Elements at Different Parts of the Power Generation Processes elements (mol/s) antimony (Sb)

arsenic (As)

beryllium (Be)

chromium (Cr)

copper (Cu)

lead (Pb)

mercury (Hg)

vanadium (V)

zinc (Zn)

inside boiler air-fired oxy-fuel oxy-fuel air-fired oxy-fuel oxy-fuel air-fired oxy-fuel oxy-fuel air-fired oxy-fuel oxy-fuel air-fired oxy-fuel oxy-fuel air-fired oxy-fuel oxy-fuel air-fired oxy-fuel oxy-fuel air-fired oxy-fuel oxy-fuel air-fired oxy-fuel oxy-fuel

(72.1%) (55.9%) (72.1%) (55.9%) (72.1%) (55.9%) (72.1%) (55.9%) (72.1%) (55.9%) (72.1%) (55.9%) (72.1%) (55.9%) (72.1%) (55.9%) (72.1%) (55.9%)

2.641 2.647 2.580 3.576 3.585 3.494 4.757 4.769 4.647 9.619 9.643 9.397 5.172 5.185 5.053 4.138 4.149 4.043 1.603 3.198 2.553 2.174 2.180 2.124 4.610 4.623 4.506

× × × × × × × × × × × × × × × × × × × × × × × × × × ×

10−4 10−4 10−4 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−5 10−5 10−5 10−2 10−2 10−2 10−2 10−2 10−2

1 (after ESP) 5.281 5.295 5.160 7.152 7.171 6.988 9.514 9.538 9.295 1.924 1.929 1.879 1.034 1.037 1.011 8.276 8.297 8.085 1.074 2.142 1.710 4.348 4.359 4.248 9.220 9.247 9.011

× × × × × × × × × × × × × × × × × × × × × × × × × × ×

10−6 10−6 10−6 10−5 10−5 10−5 10−5 10−5 10−5 10−4 10−4 10−4 10−4 10−4 10−4 10−5 10−5 10−5 10−5 10−5 10−5 10−4 10−4 10−4 10−4 10−4 10−4

2 (after FGD in air-fired) 5.281 7.942 1.548 7.152 1.076 2.096 9.514 1.431 2.788 1.924 2.893 5.638 1.034 1.556 3.032 8.276 1.245 2.426 1.074 3.214 5.131 4.348 6.539 1.274 9.220 1.387 2.703

× × × × × × × × × × × × × × × × × × × × × × × × × × ×

10−8 10−8 10−6 10−7 10−6 10−5 10−7 10−6 10−5 10−6 10−6 10−5 10−6 10−6 10−5 10−7 10−6 10−5 10−7 10−7 10−6 10−6 10−6 10−4 10−6 10−5 10−4

3

6

7

8 (recycle)

5.215 × 10−6 3.612 × 10−6

3.911 × 10−6 1.484 × 10−6

1.304 × 10−6 2.127 × 10−6

3.991 × 10−6 3.032 × 10−6

7.063 × 10−5 4.891 × 10−5

5.297 × 10−5 2.010 × 10−5

1.766 × 10−5 2.881 × 10−5

5.405 × 10−5 4.107 × 10−5

9.395 × 10−5 6.506 × 10−5

7.046 × 10−5 2.674 × 10−5

2.349 × 10−5 3.833 × 10−5

7.189 × 10−5 5.462 × 10−5

1.900 × 10−4 1.316 × 10−4

1.425 × 10−4 5.407 × 10−5

4.749 × 10−5 7.750 × 10−5

1.454 × 10−4 1.105 × 10−4

1.021 × 10−4 7.074 × 10−5

7.661 × 10−5 2.907 × 10−5

2.554 × 10−5 4.167 × 10−5

7.817 × 10−5 5.939 × 10−5

8.173 × 10−5 5.660 × 10−5

6.129 × 10−5 2.326 × 10−5

2.043 × 10−5 3.334 × 10−5

6.254 × 10−5 4.752 × 10−5

2.110 × 10−5 1.197 × 10−5

1.583 × 10−5 4.920 × 10−6

5.276 × 10−6 7.052 × 10−6

1.615 × 10−5 1.005 × 10−5

4.294 × 10−4 2.974 × 10−4

3.220 × 10−4 1.222 × 10−4

1.073 × 10−4 1.752 × 10−4

3.286 × 10−4 2.496 × 10−4

9.108 × 10−4 6.308 × 10−4

6.831 × 10−4 2.592 × 10−4

2.277 × 10−4 3.716 × 10−4

6.970 × 10−4 5.296 × 10−4

the percentages mentioned above were taken for the calculations. Table 4 presents the molar flow rates of the main trace elements at different points of the oxy-fuel processes and the air-fired case. The simulations were performed considering atmospheric pressure, and the temperature range for both the air-fired case and the oxy-fuel case with 72.1% flue gas recycle was 30−1200 °C, whereas the temperature range for the other oxy-fuel case with 55.9% flue gas recycle was 30−1350 °C. The lower point of the temperature range corresponded to the temperature of the flue gases at the exit of the processes, while the higher point corresponded to the temperature at the exit of the furnace. Four main divisions of the temperature range were defined. The temperature ranges for the analyzed cases were characterized as indicated in Table 5.

The mass balance calculations in the oxy-fuel cases is considered the amount of trace elements that would be recycled back into the boiler. Different studies have assessed trace element removal by different flue gas cleaning devices for air-fired systems.23 The Central Research Institute of Electric Power Industry in Japan (CRIEPI) estimated the inlet and outlet streams for a 500 MW air-fired power station based on the work of Yokoyama et al.23,25 The removal ratios at different stages of the process for the different trace elements were estimated based on their group classification presented by Clarke.26 The classification is based on the partitioning behavior of trace elements after combustion and are organized according to their volatility. Group 1 includes elements that are found in coarse particles, group 2 elements in finer particles, and group 3 elements not found in solids. For group 1 and 2 elements, approximately 98% removal was reported, whereas for group 3 elements, the removal corresponded to 33%. The removal estimates considered the removal from the boiler and ESP.25 These values were used in the calculations performed in this work and included in the mass balance for every trace element. In the case of B and Se, which are considered to be between group 3 and group 2, the amount recycled to the boiler was evaluated considering both 33% and 98%. According to Yokoyama et al.,25 the removal of trace elements from groups 2 and 3 could increase approximately to 99% at the exit of the stack, including removal at the FGD unit. In the oxy-fuel cases, the majority of the flue gases were recycled after water was condensed. It is possible that a larger amount of trace elements could be removed from the flue gas during water condensation. However, for the purpose of studying the impact of a higher concentration of trace elements on their speciation,

4. RESULTS AND DISCUSSION The air-fired case and the oxy-fuel case with 72.1% recycle flue gas presented the same equilibrium speciation of the trace elements. The oxy-fuel system with 55.9% flue gas recycle also showed the same speciation for temperatures below 1200 °C. For higher temperatures, most of the species remained the same but very small amounts of hydrogen containing species were formed for some trace elements, for example, gaseous NiH or AsH. It was concluded that combustion of coal in an oxy-fuel environment does not significantly affect speciation, in line with reports from different authors.5,10,27 However, the recycling of these elements increases their concentration inside the boiler.27 The concentration of the majority of trace elements in the oxy-fuel case with 72.1% flue gas recycle was approximately 4671

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Table 5. Temperature Ranges for the Analyzed Power Systems region 1 region 2 region 3 region 4

air-fired

oxy-fuel (72.1%)

oxy-fuel (55.9%)

between the exit of the furnace and the exit of the finishing reheater (1200−870 °C) between the exit of the finishing reheater and the exit of the lower economizer (870−380 °C) between the exit of the lower economizer and the exit of the air-heater (380−140 °C) between the exit of the air heater and the exit of the flue gases from the process (140−30 °C)

between the exit of the furnace and the exit of the finishing reheater (1200−870 °C) between the exit of the finishing reheater and the exit of the lower economizer (870−365 °C) between the exit of the lower economizer and the exit of the air-heater (365−155 °C) between the exit of the air heater and the exit of the flue gases from the process (155−30 °C)

between the exit of the furnace and the exit of the finishing reheater (1350−1020 °C) between the exit of the finishing reheater and the exit of the lower economizer (1020−370 °C) between the exit of the lower economizer and the exit of the air-heater (370−180 °C) between the exit of the air heater and the exit of the flue gases from the process (180−30 °C)

Figure 4. Thermodynamic equilibrium diagrams of Hg for (top) air-fired and (bottom) oxy-fuel (72.1%) with trace element recycle for the base cases. Equilibrium diagrams were edited from MTDATA.

amount of trace elements recycled back to the boiler may be overestimated because the removal from the condensation of water was not considered. This is particularly relevant in the case of Hg, as it was shown that the main form of this element at low temperatures was gaseous HgCl2, which is a water-soluble compound, and thus a larger amount would be removed in the water condensation step.5 Oxy-fuel wet recycle designs, where flue gases are recycled back to the boiler before water is

0.3% higher relative to the air-fired system, except for the case of Hg where it was doubled. This increase in concentration was due to trace element recycling and removal at different points of the process (Figure 4). The importance of oxidation by Cl to the speciation of Hg in air-fired boilers28 is acknowledged, and it is likely that such kinetically controlled reactions in the cooler regions (below 400 °C)29 will be of similar importance in an oxy-fuel boiler. However, it has to be mentioned that the 4672

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condense as silicates (Be2SiO4, CoSiO4, Ni2SiO4), the volatility increased in the oxy-fuel case. The case of zinc was different as its volatility increased with temperature, except in the range between 870 °C and 1000 °C, where it decreased. The main condensed form for this element at high temperature was the silicate form (Zn2SiO4) which changed to the sulfate form (ZnSO4) at approximately 680 °C. At high temperatures, silicate association of components such as Zn, Be, Co, and Ni may be a factor decreasing the volatility of these elements.33,34 The classification of trace element behavior during combustion presented by Clarke26 was met for all elements excluding the ones that formed silicates. Trace element partitioning behavior is therefore dependent on both operating conditions and the chemical reaction system.33 4.1. Sensitivity Analysis. The reduction in the Cl content of the coal had an important impact on the speciation of the majority of the trace elements studied in this work. In general terms, the reduction in Cl decreased the concentration of Cl-containing species, while the concentration of oxygen and S-containing species increased. A low Cl concentration was found to primarily affect the condensation temperature of the same elements that had shown the larger differences in volatility between air-fired and oxy-fuel systems (Cd, Cu, Pb). The relative volatilities of these elements in the air-fired and the two oxy-fuel systems considered were not altered, but the volatility of each of them with respect to the composition base case was reduced (Figure 5). Cl compounds were primarily found at high temperatures in the gaseous phase, which may indicate that chlorine increases the volatility of trace elements, as concluded by different researchers.10,35,36 Another element affected by the presence of a high concentration of Cl was Hg. The effect of Cl on the species of Hg emitted from air-fired boilers has been confirmed experimentally.28 This is also consistent with the predicted impact on Hg speciation found here, where it can be observed that for low temperatures, elemental gaseous Hg was less likely to be a stable form, whereas HgCl2(g) was the main species predicted to form. A decrease in the concentration of Cl reduced the temperature range at which HgCl2(g) was predominant, favoring the formation of the oxygen-containing species HgO(g) (Figure 6). The low S case presented interesting results regarding the formation of CaCO3 at low temperatures, which is thought to be important in CO2-rich environments. This compound did not form under high S conditions or any other case analyzed in the current work, and an important relationship between S and Ca was observed in terms of the formation of stable condensed species. As discussed in the literature,5,10 the formation of sulfate compounds is thermodynamically favored compared to carbonates. The concentrations of S in the molar balances for the three power systems showed a lower amount of S compared to Ca for the air-fired and oxy-fuel case with 55.9% flue gas recycle. In both cases, CaCO3 was formed. The oxy-fuel case with 72.1% flue gas recycle presented a higher concentration of S which did not allow the formation of CaCO3. Another example of this effect was the formation of condensed B4CaO7 and As2Ca3O8, which only took place in cases where the concentration of S was lower than that of Ca (Figure 7). The elements mostly affected by a lower concentration of S were Cd, Cu, Pb, and V. In the case of the former three elements, the volatility increased compared to the base case (Figure 8). The speciation of these elements was mainly affected by the amounts of Cl and S, as mentioned in the previous sections. The dependence of volatility on S among

condensed, may have concentrations as high as the ones presented in this work. On the other hand, using a low temperature ESP may allow a larger amount of trace elements to be in the condensed form, making their removal easier. A further analysis of the results was performed by determining the temperature at which the concentration of the condensed phase was greater than 50% (T50), an approach previously used in the literature.15,30,31 Table 6 presents the T50 Table 6. Condensation Temperature of 50% of Trace Elements element

air-fired °C °C °C °C

antimony (Sb) arsenic (As) beryllium (Be) boron (B)

T50 T50 T50 T50

cadmium (Cd) chromium (Cr) cobalt (Co) copper (Cu) lead (Pb) manganese (Mn) mercury (Hg) nickel (Ni) selenium (Se)

T50 < 590 °C T50 < 1160 °C T50 < 1070 °C T50 < 610 °C T50 < 700 °C condensed gas T50 < 1130 °C T50 < 60 °C

vanadium (V) zinc (Zn)

T50 < 660 °C T50 < 1170 °C

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